The objectives of this research proposal are to investigate: 1) the nature of inhibitory neurotransmitter responsible for internal anal sphincter (IAS) relaxation in response to rectoanal reflex; 2) the cellular bases underlying the myogenic tone in the IAS; 3) the neural pathways in the rectoanal inhibitory reflex; and, 4) the influence of different neuropeptides, regulatory peptides and other neurohumoral substances on IAS function. Since the IAS plays a key role in continence and defecation, its disorders underlie many clinical disorders, such as fecal incontinence and constipation. Disorders of the IAS may also be involved in a variety of anal diseases such as hemorrhoids and anal fissures. Disorders of the IAS occur more frequently in the elderly and are of paticular importance in this age group. Studies are planned in vivo and in vitro in which the activity of the IAS will be isolated from those of other muscles, such as the external anal sphincter (EAS), by simultaneous electrical and mechanical recordings. The studies dealing with the immunohistochemical localization of physiologically relevant neuropeptides, radioimmunoassay (RIA) of neuropeptides, membrane potential changes using intracellular electrical recordings, changes in the free intracellular Ca2+ will also be used. The factors which determine the basal tone will be investigated. Specifically, the nature of the inhibitory neurotransmitter for IAS relaxation, the cellular bases for intrinsic myogenic activity, neural and hormonal factors affecting the IAS fnction, and the role of different regulatory peptides on the function of the IAS will be evaluated. Most of these studies will be performed in opossums, but other animal species will also be used to determine species differences in the control of the sphincter. Some studies are planned in awake animals to understand the normal physiology of the IAS. The proposed studies should provide important information on the physiology and pharmacology of the IAS which may be useful in the understanding of pathophysiology, the effects of various drugs and the treatment of anorectal disorders.